Field of the Invention
The present invention relates to an objective optical system for endoscope.
Description of the Related Art
An endoscope is an optical apparatus that has been used in a medical field and an industrial field. Particularly, in the medical field, an endoscope is inserted into a body cavity and accordingly, an observation of inside of a body and acquiring of an image is carried out. Moreover, a diagnosis is made on the basis of images observed and images acquired. In such manner, an endoscope has been used for observation of an inside of a body and for diagnosis.
In an objective optical system for endoscope, an optical system of a retro-focus type has been adopted. In recent years, for improving an accuracy of diagnosis, a high quality of an image of endoscope has been sought. In objective optical systems for endoscope until now, improvement to a superior image quality such as by widening an angle of view, has been carried out while a lens arrangement of the retro-focus types maintained as it has been. Such objective optical system for endoscope has been disclosed in Japanese Patent Application Laid-open Publication No. Hei 4-275514 and Japanese Patent No. 5374667 Publication.
An objective optical system for endoscope disclosed in Japanese Patent Application Laid-open Publication No. Hei 4-275514 includes in order from an object side; a negative lens, a positive lens, and a cemented lens. The cemented lens includes a positive lens and a negative lens. In the objective optical system for endoscope of Japanese Patent Application Laid-open Publication No. Hei 4-275514, a curvature of field which was not corrected adequately previously, has been corrected favorably, and progress has been made in dealing with a large number of pixels of an image pickup element.
An objective optical system for endoscope disclosed in Japanese Patent No. 5374667 Publication includes in order from an object side; a negative lens, a positive lens, and a cemented lens. The cemented lens includes a positive lens and a negative lens. In the objective optical system for endoscope of Japanese Patent No. 5374667 Publication, an objective lens arrangement for endoscope of the retro-focus type has been adopted.